Dual sided processing and devices based on freestanding nitride and zinc oxide films

ABSTRACT

Thin freestanding nitride films are used as a growth substrate to enhance the optical, electrical, mechanical and mobility of nitride based devices and to enable the use of thick transparent conductive oxides. Optoelectronic devices such as LEDs, laser diodes, solar cells, biomedical devices, thermoelectrics, and other optoelectronic devices may be fabricated on the freestanding nitride films. The refractive index of the freestanding nitride films can be controlled via alloy composition. Light guiding or light extraction optical elements may be formed based on freestanding nitride films with or without layers. Dual sided processing is enabled by use of these freestanding nitride films. This enables more efficient output for light emitting devices and more efficient energy conversion for solar cells.

REFERENCE TO PRIOR APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/198,484, which was filed on Nov. 5, 2008, whichis herein incorporated by reference.

BACKGROUND OF THE INVENTION

Semiconductor devices are used in a wide variety of applications.Silicon has been used as the substrate material of choice due to costand availability of single crystal silicon wafers. Driven by themicroelectronic industry silicon wafer production has enabled theeconomical use of larger wafers with greater than 12 inch diameter. Thelow cost of silicon allows thick (1-2 mm) single crystal silicon wafersto be used without a secondary substrate for handling and processing.This enables the formation of large area die, which can be processedwithout the need for transfer substrates or wafer bonding. Additionally,the properties of silicon permit wafers to be doped so that 3dimensional devices (via planar processing) can be created takingadvantage of the wafer conductivity. For some devices it is desirable toutilize thinning techniques to reduce the overall thickness of thesilicon to tens of microns to improve thermal performance. Althoughsilicon has been the dominant material for most microelectronicapplications there are other materials, which have desirable propertiesand advantages over silicon in the areas optoelectronics, solar cellsand power devices. However, heretofore there has not been an economicalsolution to using these materials without growing them or attaching themto secondary substrates.

Silicon also has limitations with regard to operating temperature aswell as other critical material parameters; this limits silicon'susefulness in optical, power and high-frequency applications. Nitridealloys and oxide alloys have several properties, which are superior tosilicon ranging from higher thermal conductivity to biocompatibility.Unfortunately, nitrides are not available in wafer form at a reasonableprice or reasonable quality. Even if such wafers were to becomeavailable, costly growth, dicing, and thinning techniques would berequired to create useful devices. In most cases, devices with anoverall thickness between 20 and 150 microns are desired from a thermalimpedance, packaging, and optical efficiency standpoint. As such, filmbased processing offers several advantage over bulk wafer approaches. Asthe solar industry has discovered film and foil based processing is muchmore cost effective than wafer based approaches if high resolutionlithography is not required. The need exists for cost effective thinnitride films, but also for the ability to process both sides of thesefilms for the formation of waveguides, edge emitters, and symmetricdevice structures.

In copending patent applications, it has been shown that stacked LEDs,solar cells, and other optoelectronic devices based on freestandingnitride films offer significant advantages from the standpoint ofcurrent spreading, functionality, color combining, and peak drivelevels. Nitride alloys and zinc oxide alloys offer a unique set ofproperties with regard to optical, electronic and optoelectronicdevices. Further, it has been shown that freestanding nitride films,which exhibit alpha less than 1 cm-1 throughout the visible spectrum,can be created with thicknesses ranging from 20 microns to over 100microns, which also exhibit resistivities less the 0.05 ohm cm and athermal conductivity approaching 200 W/m/K with sufficient areas toenable device fabrication. This unique combination of properties at athickness and area sufficient for handling and subsequent processingpermits unique processing techniques including dual sided processing.

As disclosed previously, the use of freestanding nitride filmscontaining MQW structures can be used to create isotropic light sourcesand the use of stacked freestanding nitride films with different MQWstructures can be used to create isotropic white light sources. Thecombination of these devices with solid wavelength conversion materialshas also been disclosed in a pending patent application. However tofully take advantage of these unique attributes, the need exists for lowcost processes and materials, which can be used to create devices basedon freestanding nitride films.

In particular, the need exists for techniques whereby processingincluding but not limited to epitaxial growth, deposition, laserpatterning, printing, and various interconnect means can be done on oneor both sides of freestanding nitride films for the formation of 3dimensional devices and dual side processes. Zinc oxide alloys can beused to create 3 dimensional devices based on similar materialproperties to nitrides. While native ZnO wafers have been fabricated,their cost is restrictive and they lack a suitable/stable p-dopant whichlimit their applications. The combination of nitride alloys and zincoxide alloys can be used to create a wide variety of high performanceelectrical and optoelectronic devices. Therefore there is a need foreconomical processes and means to create 3 dimensional devices based onnitride alloys and/or zinc oxide alloys and combinations of the twomaterials sets. In this manner, a wide range of hybrid devices can berealized.

While wafers offer significant advantages to high-resolution circuitryas used in microprocessors and memory devices, which require up to 50masking steps, many applications do not require high-resolutionlithography steps. In some cases, processing round wafers is actually adisadvantage or limitation to be overcome. As an example, solar cellsare made in ribbon and large area formats. Alternately, liquid crystaldisplays contain semiconducting active-matrix backplanes are grown onlarge glass plates that measure four to 6 feet in dimension. Unlike thesemiconductor industry, the thick film industry tends to use squaresubstrates, which are more compatible with the printing techniquestypically used to form patterned conductors and dielectrics. Square,ribbon and tape based substrates have less edge loss than yields onround wafers.

In addition, the formation of non-circular devices offers severaladvantages with regard to device performance. There is the potential toreduce stress and enhance extraction efficiency where the edges of thedevices are aligned to natural cleavage planes. Triangular shaped diecan be effectively utilized to form recycling light cavities, which canenhance radiance to light emitting diode light sources. In copendingpatent applications methods of forming large area nitride layers basedon the removal of thick HVPE epitaxial layers from sapphire substratesare shown. The resulting freestanding foil substrates enable increasedflexibility in packaging and device design. The need therefore existsfor methods and articles, which take advantage of these freestandingnitride films and provide techniques for processing into devices, whichtake advantage of the material, geometry, thermal mass, and flexibilityof these freestanding nitride films.

These freestanding films offer several advantages over nitride layerswhich are transferred to a secondary substrate via waferbondingtechniques, and over films which remain on their growth substrate likesapphire or silicon carbide, and over diced and polished nitride wafers.

In the case of waferbonded films, stresses created during the originalgrowth process are transferred via the wafer bonding process. Also theresulting structure suffers from poor thermal performance, thermalexpansion mismatch which limits operating range. Typically the thesenitride layers are thin (˜3 microns) which can impact packagingprocesses such as wirebonding to electrical contacts on the device. Thisis due to the fragile nature of the thin nitride film, requiring reducedbonding forces to prevent cracking of the nitride layer. This lowersyield in final device fabrication. The lower permissible operatingtemperature range due to the use of secondary substrates used inwaferbonding not only limits device operation but also prevents the useof robust materials such as glass frits and fired contacts that canenhance packaging reliability but require high temperature processing.

With these prior art methods, nitride films left on their growthsubstrate must be thin to prevent bowing either at room temperature orat high temperature growth conditions. The low thermal conductivity ofsapphire in particular limits device performance and also typicallyrequires wafer thinning processes, which increase costs. Also, theinability to create a vertical structure for non-conductive growthsubstrates like sapphire limit the ability to form vertical structuresor stack nitride films, which can result in new and novel devices.

Another prior art technique to form freestanding nitride substrates isto grow thick nitride layers on secondary substrates, followed by dicingand polishing to eliminate the secondary substrate. This has proven tobe a very costly process and by necessity has size limitations. Further,defects introduced via dicing and polishing and the inclusion ofstresses due to the dicing processes limit the yield and viability ofthis approach. In these prior art techniques dual sided processing isimpossible or very difficult. However, dual sided processing could offerunique advantages and lead to new types of devices.

It would be desirable to have the ability to grow a variety ofstructures on a wafer level and then form a freestanding nitride film,which can be further processed on one or both sides. This could offersignificant packaging and device flexibility. Alternately, the abilityto grow directly on freestanding nitride films could offer many benefitsincluding but not limited to; lower stress growth, dual side processing,low thermal mass and flexible substrates.

In copending patent applications we have shown where freestanding thick(15 to 150 micron thick) nitride layers can be formed with sufficientarea and low cost. These layers have an epitaxial ready surface, can bestacked, and can be stress relieved either by allowing the films to bowor be subjected to high temperature annealing processes, once free ofthe growth substrate. In addition, features and additional layers can beadded to one or both sides of the freestanding films. These freestandingnitride and oxide films can be processed on both sides at temperaturesover 700 degrees C. enabling the use of thick film processes developedby the solar industry. The use of high temperature thick film processesto form LEDs, diodes, optoelectronic devices, Mems, solar cells andother semiconducting devices could benefit by dual sided processing. Inaddition, it would be possible to take advantage of the low thermal massof thin freestanding nitride foils to enhance the epitaxial devicegrowth process.

SUMMARY OF THE INVENTION

This invention discloses methods and articles based on large areafreestanding nitride films. These freestanding nitride films enable dualsided processing, improved crystal quality, reduced growth cycles andnovel packaging not possible with more conventional template, transferand bulk wafer approaches. Several attributes of freestanding nitridefilms are important aspects of this invention including flexibility, lowthermal mass, thickness, crystal quality, epitaxial-ready surface, andlow stress. The intent of this invention is to disclose methods andarticles which take advantage of these attributes of these freestandingnitride foils in LEDS, Laser diodes, solar cells, power devices, rfdevices, and 3D semiconductor devices. More preferably, the use of thesefilms with thickness greater than 10 microns in semiconductor devices isdisclosed. Even more preferably freestanding nitride films withthickness greater 20 microns but less than 150 microns is disclosed.Most preferred is freestanding nitride films with a thickness between 20and 70 microns and an area greater than 1 mm2. These freestandingnitride films can be coated, handled, segmented, printed on, grown on(epitaxially and non-epitaxially) and processed at elevatedtemperatures. This invention also discloses the formation ofoptoelectronic devices on freestanding nitride films greater than 10 μmthick with epitaxially grown transparent conductive layers greater than5000 Angstroms. By using epitaxially grown transparent conductiveoxides, thicker layers with lower absorption, better current spreading,ESD protection, and lower contact resistance can be formed than usingnon-epitaxial approaches. The use of epitaxial growth of MOCVD doped ZnOalloys on one or both sides of a freestanding nitride films is apreferred embodiment of this invention.

In addition, the use of thick transparent conductive oxides to enhancethe structural integrity of the freestanding nitride films for improvedhandleability is also disclosed. Preferably, the formation of greaterthan 5000 Angstroms of a transparent conductive oxide (TCO) on one orboth sides of a freestanding nitride film is preferred embodiment ofthis invention. Even more preferred, is the formation of greater than5000 Angstroms of a TCO on both sides of a freestanding nitride foil.These layers can be used to inhibit fracture as well as provideinterconnect to the underlying device structure.

The use of high temperature annealing steps for the freestanding filmsafter removal of the growth substrate is an embodiment of thisinvention. The use of high temperature annealing steps after removal ofthe growth substrate and at least one side coated with a transparentconductive oxide is also disclosed.

The use of these films to form semiconductor devices is an embodiment ofthis invention. In this manner very low Vf devices can be formed withoutthe need for rapid thermal annealing.

By eliminating the need for rapid thermal annealing, high reflectivityohmic contacts can be readily formed. Typically complex metallizationsteps are required due to the degradation typically induced by the rapidthermal annealing steps. The epitaxial growth of the transparentconductive layer may be on one or both sides of the large areafreestanding nitride film.

By epitaxially growing a thick transparent conductive layer, severaldevice parameters are improved significantly. These include mechanicalintegrity, turn-on voltage, contact resistance, ESD stability, and theability to form ohmic contacts.

Optoelectronic devices, not limited to, LEDs, laser diodes, solar cells,biomedical devices, thermoelectrics, and other optoelectronic devicesthat are fabricated on these freestanding nitride films are embodimentsof this invention. The growth of these devices on freestanding nitridefilms is a preferred embodiment of this invention. The low thermal massof the freestanding nitride films allow for the use of rapid thermalprocessing methods in epitaxial device growth in particular.Freestanding nitride films are flexible, single crystal, hightemperature, chemically stable, and exhibit low thermal mass, whichmakes them ideally suited for a wide range of epitaxial andnon-epitaxial processes. Not only do freestanding nitride films provideenhanced growth substrates for transparent conductive oxides (TCOs) butthey also can be used to enhance the quality of a wide range ofsubsequent epitaxial growths including but not limited to nitridealloys, silicon, antimonides, germanium, and other importantsemiconductors. In particular, freestanding nitride films with athickness between 20 and 150 microns are flexible and exhibit very lowthermal mass, which are shown to enhance subsequent epitaxial growthsteps. The flexible nature of these films, can reduce the stress andalso improve crystal quality during subsequent epitaxial growths. Thiscan enhance the optical properties of the TCOs. In addition, the lowthermal mass of the freestanding nitride films allows for hightemperature processing not permissible with conventional nitridefabrication processes. These freestanding nitride films can befabricated to exhibit absorption coefficients less than one percentimeter over the operational range of the target device or be made tobe strongly absorbing depending on the alloy composition.

In addition, the refractive index of the freestanding nitride films canbe controlled via alloy composition. The formation of light guiding orlight extraction optical elements based on freestanding nitride filmswith or without layers is an embodiment of this invention.

Alternately, the mechanical properties of the freestanding films can beuseful for applications such as bimorphs, unimorphs, cantilevers,micro-actuators and other MEMS type devices. The formation of layers onone or both sides of the freestanding nitride films for mechanicaland/or acoustomechanical applications is disclosed.

A preferred embodiment of this invention are nitride alloys whichcontain but are not limited to AlGaN, InGaN, AlInGaN, GaN, AlN, InN,InAlN as well as P and As alloys typically referred to as dilutenitrides. The modification of the resulting freestanding films viaetching, mechanical means, laser, and other techniques known in the artto reduce thermal conductivity, create optical structures, formcomposites for optoelectronic, thermoelectric, solar, and/orpiezoelectric devices is an embodiment of this invention.

With regard to transparent conductive layers, most preferred, is the useof oxide alloys of zinc formed via MOCVD as transparent conductivelayers.

The use of dopants to impart luminescent, n doping, p doping,semi-insulating and degenerative properties are also included in theconfigurations listed. In particular, the use of Al, Ga, and Mg to formhighly conductive transparent layers as dopants or alloys to ZnO is anembodiment of this invention.

The use of freestanding nitride films as a growth substrate to enhancethe optical, electrical, mechanical and mobility of thick transparentconductive oxides is an embodiment of this invention. In the prior artthere have been efforts to grow low defect density GaN on silicon versussapphire. This has not been successful resulting in much higher defectdensity films. In this invention it has been found that the quality ofAl doped ZnO grown on freestanding nitride films is inherently higherthan when the nitride films is still attached to a sapphire substrate.The constrained nature of the nitride on sapphire or any other growthsubstrate including AlN negatively affects the subsequent growth of theZnO layers. This translates into better ohmic contacts and betteroptical properties, which in turn lead to better device performance. Inaddition the use of the excess gallium formed during laser separation tofurther dope the transparent conductive oxide is beneficial to theresulting properties of the film. The ability to epitaxially grow highquality transparent conductive layers with very good optical propertiesalso enables the use of high temperature and/or high energy processes.As an example laser welding of silver and aluminum ribbon direct to 1.5micron thick ZnO has been demonstrated with very low contact resistanceand is an embodiment of this invention.

In addition, epitaxial growth on freestanding nitride films allow forthe growth of high quality highly doped degenerative layers which can beused for improved current spreading and ohmic contacts to a wide varietyof optoelectronic devices. Unlike amorphous and polycrystalline growths(typically used to deposit these layers) the MOCVD process used hereinprovides not only high electrical conductivity, but also provides verylow optical absorption losses.

In addition, the use of zinc oxide MOCVD layers on freestanding galliumnitride creates a tough outer skin, which reduces cracking due tocrystal plane differences between the two materials. The use of zincoxide MOCVD layers on one or both sides of the freestanding nitridelayers is an embodiment of this invention. The zinc oxide exhibits alower refractive index then gallium nitride. As such, the use of zincoxide layers as index matching, cladding layers, and other opticalelements is an embodiment of this invention.

Thick freestanding nitride films allow for unique packaging and devicefabrication. In particular, freestanding nitride layers can be easilycut, patterned, and perforated using laser and other actinic radiationsources. The formation of multiple layer devices is an embodiment ofthis invention. This includes but is not limited to, multi-layerinterconnects, heat sinks, micro-optical devices, LED arrays, and solarcells.

The use of degenerative highly doped transparent conductive layers, likebut not limited to aluminum doped zinc oxide, enable the formation ofstacked layers which can be electrically connected either across theentire area of each layer or spatially selectively attached. The abilityto form 3 dimensional devices and circuits using freestanding nitridefilms and transparent conductive layers is an embodiment of thisinvention.

As current density increases on devices like LEDs, current spreadingbecomes a limiting factor in device performance especially for largearea devices. In co-pending patent applications, freestanding nitridefilms on which MQWs are grown can be stacked and interconnected. Byusing the highly transparent conductive layers described herein,emitting volumes can be formed. Rather than have a very large die withlarge variations in drive currents across the die, stacked die can notonly improve the current spreading across a given area but increase thelumens/etendue or radiance of the device. Similarly, this same techniquecan be use to form more efficient and concentrated solar cells andelectronic devices such as IGFETS. With nitrides, current spreadinglimitations tend to be more severe than thermal effects, thereforevolume emitters tend to be more efficient than surface emitters. Lowabsorption and high quality ohmic contacts are required to enable volumeemitters like these. The use of epitaxially grown transparent conductivelayers on freestanding nitride films stacked to form volume emitters,absorbers, and electrical devices is an embodiment of this invention.

Geometry plays a critical role in volume emitters versus surfaceemitters. Here is a simple example: A typical nitride LED less than 10microns in thickness and 300 micron×300 micron in area has an emittingarea equal to 0.09 mm². If the same size die is 100 micron thick thereis more emitting area, 0.12 mm², on the sides of the die than the topsurface. There are a number of advantages to this configuration. It iswell known that recombination losses are minimized directly underelectrical contacts on LEDs. Utilizing large reflective contacts on thetop and bottom of the freestanding nitride LED enable very highextraction efficiency out the sides of the die This extractionefficiency is further enhanced by the tendency for a substantial portionof the light generated within the active layer to be waveguided to theedges. The use of thick freestanding nitride films to create LEDs inwhich the output area of the surface normal to the active layer plane islarger than the output area of the surface parallel to the active layerplane is an embodiment of this invention. Further, the use of stacks offreestanding nitride films to create emitter or absorbing volumes totake advantage of the increased side output area is an embodiment ofthis invention. The formation of opaque reflective contacts on bothsurfaces parallel to the active layer plane to further increase thermalcooling and current injection and permit the light to exit or enter viathe surfaces normal to the active layer plane are embodiments of thisinvention.

The thickness of the freestanding nitride film is a critical element inoverall device performance. While a reasonable thickness is required toenable: handleability, permit low defect density, and optimum devicegeometry, there are several device performance attributes, which requirethe freestanding nitride film to be as thin as possible. Theseattributes include cost, thermal impedance, series resistance, ease informing high resolution vias and interconnects, flexibility, and lowinternal absorption losses. Another key attribute is the ability torapidly change temperature during growth processes. Nitride templatesgrown on non-native growth substrates significantly limit allowablegrowth conditions due to bowing and cracking due to the mismatch ofthermal properties of the two materials. Freestanding nitride films notonly eliminate these effects but also enable the use of rapid thermalprocessing steps for the formation of quantum wells (QW), multiplequantum wells (MQW), superlattice and other epitaxial structures whichrequire rapid large transitions in reactor growth temperatures. Manynitride devices require a large number of layers to be grownsequentially in the reactor. Therefore, reduction in the growth time ortransition times of each layer can significantly impact overall devicecost. Freestanding nitride films not only have inherently lower thermalmass than the more conventional wafer based approaches, nitrides havelower specific heats than sapphire, silicon, and SiC. This can enablethe use of higher growth rates while maintaining layer composition andthickness control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts the prior art of a three dimensional device based onstacked silicon chips. FIG. 1 b depicts a prior art process of forming alarge area diode.

FIG. 2A depicts a process of this invention to fabricate devices using afreestanding nitride film with layers on both sides of the freestandingfilm. FIG. 2B depicts a process of the present invention to formmechanical structures in the freestanding nitride layers to createextraction elements, optical elements, scribe boundaries, electricalisolation, etc.

FIG. 3A depicts partial liftoff and other stress relief techniques forseparation of thick epitaxial layers from growth substrates of thepresent invention. FIG. 3B depicts a plan view of a preferred laser scanpattern process of the invention to liftoff the freestanding GaN fromthe growth substrate.

FIG. 4A depicts backside deposition during growth of the epitaxial layerof the present invention. FIG. 4B shows the epitaxial layer aftercleaning the backside and edges of the overgrowth. FIG. 4C shows the useof a special platen to reduce bow during laser liftoff processing. FIG.4D shows the lifted off freestanding nitride layer with patternedstructures. FIG. 4E shows the patterned freestanding epitaxial film withovercoating. FIG. 4F shows the epitaxial film separated into individualdie.

FIG. 5A depicts a dual sided epitaxial solar cell of the presentinvention. FIG. 5B shows a side view of a solar cell using theinvention.

FIG. 6A depicts a vertical LED fabricated using the methods of theinvention. FIG. 6B shows how spatially varied features can improvecurrent spreading of the high brightness LEDs.

FIG. 7A depicts a means of making a broadband light source using stackedfreestanding LEDs of the present invention. FIG. 7B shows how to use thepresent invention to create LEDs or solar cells with variable wavelengthsensitivity on the same die.

FIG. 8A depicts an EELED formed using freestanding films and dual sidedprocessing of the present invention. FIG. 8B shows how to fabricatepartial mirrors for EELEDs, Lasers, etc with the present invention.

FIG. 9A depicts restriction of output of stacked volume emitter of thepresent invention. FIG. 9B shows a volume emitter fabricated with thepresent invention with a recycling cavity and optical element on theoutput.

FIG. 10A depicts at least one linear emitter containing an addressingmeans of the present invention. FIG. 10B shows interconnect means tolinear display.

FIG. 11 depicts a volume emitter combined with at least one solidluminescent emitter of the present invention.

FIG. 12 depicts a LED powder of the present invention.

FIG. 13 depicts a light source using LED powder of the presentinvention.

FIG. 14A depicts temperature control of freestanding nitride filmsduring processing. FIG. 14B depicts means of capturing freestandingnitride films to achieve dual sided processing.

FIG. 15A depicts a low thermal inertia mounting. FIG. 15B shows processtemperature ramps.

FIG. 16 graphs thermal time constant T of various substrates as afunction of h.

FIG. 17A depicts typical LED growth cycle. FIG. 17B shows the use ofgrowth interruptions to improve device performance.

FIG. 18A depicts a rapid thermal multi chamber deposition method basedon freestanding nitride films. FIG. 18B shows a freestanding nitridefilm.

FIG. 19A depicts dual sided freestanding nitride film with multipledepositions. FIG. 19B depicts interconnect means to dual sided device.

FIG. 20 depicts device growth within freestanding nitride films.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A depicts a typical prior art three-dimensional device based onstacked silicon chips. Thinning is used to reduce the device thicknessfor both via formation, overall device thickness and thermalperformance. This technique is used especially for memory devices toincrease the amount of memory that can be contained within a givenfootprint. Memory layers 1, 3, and 5 are typically freestanding siliconfilms which are physically and electrically connected via interconnects2. In this case, a via 4 may be used through the freestanding siliconfilms such that a high level of interconnect is created. However, thethinning processes used (polishing, etching, etc) negatively impactoverall process yield and ultimate device cost.

FIG. 1B depicts a prior art technique to form a large area diode byusing n doped layer 7 and p doped layer 8 with contacts 6 and 9. Theresulting PN junction can be formed by growing n doped layer 7 or pdoped layer 8 on the other layer or may be formed by waferbonding afreestanding n doped layer 7 to a p doped layer 8. In this manner verylarge area devices can be formed. However, waferbonding adds costlyprocess steps and impacts yield and device cost.

FIG. 2A depicts a freestanding nitride film 11 of the present inventionwith top or first layer 10 on the upper surface of the nitride film andbottom or second layer 12 on the lower surface of the nitride film.Freestanding nitride film 11 may consist of nitride alloys and/or zincoxide alloys. Freestanding nitride film 11 most preferably consists of anitride alloy. Even more preferred, the freestanding nitride filmconsists of GaN. Freestanding nitride film 11 may be doped, undoped andsemi-insulating. Polar, non-polar, and semi-polar orientations of thenitride layer may also be used. Most preferred are freestanding nitridefilm 11 consisting of polar GaN uniformly doped with Si. Dopingconcentrations between 10¹⁶ cm⁻³ and 10²⁰ cm⁻³ are an embodiment of thisinvention. A freestanding nitride thickness between 10 and 150 micronsis a preferred embodiment of this invention. Most preferably thicknessesbetween 20 and 70 microns is disclosed to maintain reasonableflexibility while offering sufficient thickness to reduce defect densitybelow 10⁸ cm². In a preferred method, the formation of top layer 10 isdeposited via a deposition process including, but not limited to MOCVD,HVPE, MBE, CVD, sputter, evaporative, spincoating and other depositiontechniques as known in the art, on freestanding nitride film 11. Toplayer 10 may consist of but not limited to oxides, nitrides, silicon,antimonides, metals, dielectrics and other semiconductor materials.Preferred is epitaxial growth of top layer 10 via MOCVD, HVPE, MBE, CVDor other deposition method on freestanding nitride film 11. Bottom layer12 may also consist of but not limited to oxides, nitrides, silicon,antimonides, metals, dielectrics and other semiconductor materials.Preferred is epitaxial growth of bottom layer 12 via MOCVD, HVPE, MBE,CVD or other deposition method on freestanding nitride film 11. Toplayer 10 and bottom layer 12 may be deposited separately or at the sametime. Top layer 10 and bottom layer 12 may be uniform or structured,including multilayered, superlattices, quantum dots, or composites. Apreferred embodiment of this invention is the epitaxial growth ofsemiconductor layers for use in devices including but not limited to,LEDs, EELEDs, laser diodes, solar cells, photoelectrochemical cells, rfdevices, and power devices in either or both top layer 10 and bottomlayer 12. Multiple coating steps for either or both top layer 10 andbottom layer 12 may be added to create the desired device structure. Asan example, on freestanding nitride film 11, a first top layer 10 may beepitaxial grown via MOCVD consisting of at least one InGaN/GaN quantumwell layer, followed by a p doped AlGaN barrier layer, followed by a pdoped GaN layer, followed by an additional top layer electrical contactlayer 10 consisting of a TCO consisting of but not limited to AZO, ITO,IZO, GIZO, GAZO, and other conductive oxides. In this example, bottomelectrical contact 12 may optionally be grown on the other side offreestanding nitride film 11. Alternately, metal electrical contactslayer may be added to top layer 10 and bottom contact 12 to enhancecurrent spreading. In this manner a freestanding nitride film based LED,solar cell, rf device, or power device can be formed without anyadditional support layers. A preferred embodiment is a device in whichthe last top layer 10 and last bottom layer 12 consists of Al doped ZnOwith an Al doping concentration greater than 10¹⁹ cm⁻³ and a thicknessgreater than 3000 Angstroms grown via MOCVD on freestanding nitride film11. The resulting TCO layers exhibit lower absorption losses whilemaintaining high electrical conductivity than amorphous, sputtered, orother non-epitaxially grown layers in thick layers. Layer thickness iscritical for ESD and other considerations. The growth of solar celllayers in top layer 10 and/or bottom layer 12 on freestanding nitridefilm 11 is also a preferred embodiment. Even more preferred is thegrowth of at least one solar cell layer on top layer 10 which isresponsive to short wavelengths of the solar spectrum and the growth ofat least one solar cell layer on bottom layer 12 which is responsive tolonger wavelengths of the solar spectrum. As an example, a InGaN/GaNsolar cell junction is grown via MBE for the first top layer 10 followedby a second top layer 10 consisting of but not limited to AZO or othertransparent conductive oxide, this is then followed by a Si, Ge, orother low bandgap solar cell junction grown for bottom layer 12. In thiscase the order of growth is important because this approach overcomesthe issue of degrading the lower temperature low bandgap junctions whenhigher temperature nitride solar cells layers are grown on low bandgapmaterials as performed in prior art approaches. In prior art approachesa solar cell is formed by forming a PN junction in silicon (longwavelength sensitive) and then growing a nitride PN junction on top ofthe silicon. However, the nitride processes require higher temperaturethan the silicon formed PN junction such that the subsequent processingcan degrade the silicon PN junction performance. Using the process ofthis invention thin flexible high efficiency solar cells can be createdof freestanding nitride films. Another advantage is that thefreestanding GaN is largely transparent to all wavelengths outside ofthe active region, which provide for higher optical efficiency. Inanother embodiment of the invention, top layer 10 first consists of anAlGaN buffer providing isolation of the underlying freestanding nitridefilm 11 from subsequent growths. The flexible, high temperaturecompatible, single crystal nature of the freestanding nitride film 11provides an ideal growth substrate for improving subsequent growths oftop layer 10 and bottom layer 12 which is not permissible in prior artapproaches.

In another embodiment, freestanding nitride film 11 may be stillattached to a growth substrate consisting of, but not limited to,sapphire, aluminum nitride, silicon carbide or other growth substratesas known in the art. Structures then may be grown on the exposed nitridesurface without affecting the side common to the growth substrate.

The formation of devices via printing and epitaxial growth on either orboth top layer 10 and freestanding nitride film 11 includinginterconnect, devices or micro mechanical structures prior to removal ofthe growth substrate is an embodiment of this invention.

Both top layer 10 and freestanding nitride film 11 are then removed fromthe growth substrate using a separation method including but not limitedto laser liftoff, chemical means, and/or mechanical means as known inthe art. Top layer 10 may consist of metals, ceramics, organics andcomposite materials. After removal of the growth substrate bottom layer12 is then deposited via deposition techniques including, but notlimited to, MOCVD, HVPE, MBE, CVD, sputtering, evaporative, spincoating,and other deposition techniques as known in the art.

A preferred embodiment of this invention is the epitaxial growth ofdoped ZnO alloys for top layer 10 and bottom layer 12 on a freestandingnitride film 11 consisting of nitride alloys. Even more preferred is theepitaxial growth of degenerative doped ZnO alloys with dopantconcentration greater than 10¹⁸ cm⁻³ and thickness greater than 5000angstroms. In the case of laser liftoff processes on nitride alloys, theincorporation of excess gallium, indium, aluminum, as well as otherelements during the epitaxial growth of the ZnO alloys for bottom layer12 is facilitated. This eliminates the need for cleaning steps and canbe used to enhance the ohmic contact formed between the two layers.

Patterning of freestanding nitride film 11 prior to separation from thegrowth substrate to form excess gallium, indium, aluminum and as well asother elements to enhance extraction, enhance ohmic contact, or tosegment devices on freestanding nitride film 11 is an embodiment of thisinvention. Utilizing this process it has been demonstrated that areduction in the forward voltage (Vf) on LEDs can be attained due toimproved ohmic contact being formed using a thick epitaxially grown toplayer 10 and bottom layer 12 on freestanding nitride film 11. While thecomplete mechanism of the improvement is unknown, reduced stress andcrystal quality of the epitaxially grown bottom layer 12 due to growthon freestanding nitride film 11 is believed to be a major factor in theenhanced performance. The diffusion of excess gallium, aluminum, indium,(but not limited to these elements) created by the laser liftoff anddissociation of the freestanding nitride film 11, during epitaxialgrowth of bottom layer 12, also contributes to enhanced final deviceperformance. This enhanced performance is not attainable with prior artmethods using nitrides on secondary substrates. A preferred embodimentof the freestanding nitride film 11 of substantially gallium nitridealloys where said freestanding nitride film is greater than 10 micronsof thickness and preferably less than 120 microns thickness and asurface area greater than 1 mm2, which is utilized in processing. Evenmore preferred for economical processing is a freestanding nitride film11 which is a substantially gallium nitride alloy with greater than 30microns of thickness and less than 80 microns thickness and an areagreater than 1 cm². Also preferred, the area is not greater than 30 cm²and even more preferably not greater than 10 cm². This minimizesstresses, bowing, process variations while still allowing a large numberof devices to be processed per film.

Freestanding nitride film 11 may be n doped, p doped, semi-insulating,and undoped. Typical dopants include but are not limited to Si, Fe, Mg,and Zn. These dopants may be uniformly, stepwised or gradiently doped.The selection of top layer 10 and/or bottom layer 12 such that theirrefractive indices are lower or higher than freestanding nitride film 11to enhance extraction or form a waveguiding structure is an embodimentof this invention.

Alternately, top layer 10 and bottom layer 12 may be formedsimultaneously on freestanding nitride film 11. In this case,freestanding nitride film 11 is removed from any growth substrate priorto formation of top layer 10 and bottom layer 12. Layer formation mayconsist of, but not limited to, dip coating, MOCVD, LPE, CVD, HVPE,sputtering, evaporative, and other coating techniques as known in theart. More preferred is the use of hot wall MOCVD such that a uniformepitaxially grown layer is formed on all surfaces of freestandingnitride film 11. The removal of the overcoated edges via mechanical,chemical, laser, and other etching means to isolate top layer 10 frombottom layer 12 is an embodiment of this invention. More preferred isthe partial scribing of the freestanding nitride film 11 via laserscribing. This permits the film to be separated into device chips usinga cleaving process, which provides a clean edge to the nitride chip.This eliminates shorting of the devices due to contaminants createdduring the laser process coating the edges of the chips.

FIG. 2B depicts an n-doped freestanding nitride film 16 with at leastone active region 15 and p doped layer 14 and top layer 13 on top. Atleast one active region 15 may include PN junctions, heterojunctions,single and multiple quantum wells, and solar cells. At least one activeregion 15 and/or p doped layer 14 and/or top layer 13 may be grown priorto or after separation of the growth substrate.

The thickness of n-doped freestanding nitride film 16 enables theformation of features 17. Features 17 can be formed via etching,mechanical means, lithographically, and laser machining processes. Theuse of scribe lines 17 for enhanced extraction, dicing of the individualdie, formation of directive optical elements, formation of optical,thermal, and electrical isolation is an embodiment of this invention.Most preferred is the use of laser machining processes to form features17. Most preferably, the use of UV diode pumped lasers with anadjustable shaped spot to form features 17. By adjusting the spot shape,direction of scan and spacing a wide range of features can be created.Formation of features 17 can be done after and/or before removal of thegrowth substrate for the freestanding nitride film 16.

FIG. 3A depicts an epitaxial film 19 grown epitaxially on a growthsubstrate 18. While a wide range of growth substrates 18 exist, not evennative substrates are stress free. The process of making a functionaldevice requires the use of dopants and alloys to create a desiredresult. As an example, a typical MQW LED structure would consist of Sidoped Gallium nitride, with 5 periods of InGaN and GaN layers to definethe multiple quantum wells with thickness ranging from 10 s to 100 s ofangstroms, followed by AlGaN barrier a few 100 Angstroms thick, followedby a Mg doped GaN up to a few 1000 Angstroms thick. All these layersinduce stresses and can induce bowing of the overall wafer. Thereforewhat is needed are methods, which mediate the effects of the bowing.These stresses not only can cause bowing but may cause efficiency andspectral performance changes.

It is therefore a preferred embodiment of this invention that the growthsubstrate 18 be removed with the minimum amount of damage or stressimparted to the epitaxial film 19. As previously disclosed, the use ofscanned laser pulses 20 can be used to gently remove an epitaxial film19 from growth substrate 18. Laser radiation 20 is transmitted throughgrowth substrate 18, which is substantially transparent to laserradiation 20. By selecting the appropriate wavelength and focus, a smalllocalized dissociation 23 of the epitaxial film 19 can be created. Inthe case of gallium nitride, this typically leads to the formation ofgallium metal and nitrogen. The use of a very small pulse preferred lineshaped allows the simultaneous formation of extraction elements and/orstress relief surfaces in the epitaxial film 19. Care must be taken touse a sufficiently small enough pulse in relation to the thickness ofepitaxial film 19 to prevent cracking.

An embodiment of this invention is the use of DPSS laser with at leastone dimension of the spot less than 10 microns. Even more preferable isthe use of a line shape which is less than 5 microns in width andgreater than 100 microns in length. In the case of thick epitaxial films19, an alternate embodiment of this invention is the use of scannedlaser pulses 22 to prescribe the growth substrate 18. Because thesapphire is typically heavily stressed it is possible to induce a stresscrack 24 in the growth substrate 18 without fracturing the epitaxialfilms 19 using this technique. Not only does this stress crack reducethe bowing of the overall wafer, it also allows for improved liftoffbecause the edges of the wafer are already lifted off.

Unfortunately, as stated earlier, some level of stress is present in anyepitaxial film 19, which leads to bowing of both the growth substrate 18and epitaxial film 19. In the case of thick (greater than 10 microns)epitaxial film 19, bowing is typically between 10 and 100 microns acrossa 2 inch wafer. Using a variety of techniques, bowing can be adjustedfor a given temperature. If there is too much bow at room temperature,separation techniques can lead to cracking of epitaxial film 19. If thebow is too large at growth temperature, it may be difficult to hold thewafer in the reactor.

The use of this temperature dependent bowing effect to adjust the growthconditions of devices on epitaxial film 19 is included as an embodimentof this invention. In general, methods are required to deal with thestress created during epitaxial growth of thick epitaxial film 19 on anygrowth substrate 18. As epitaxial film 19 thickness increases, epitaxialfilm 19 begins to behave more and more like a bulk material. Epitaxialfilms 19 of gallium nitride with thicknesses approaching 100 micronstypically exhibit bows in excess of several hundred microns on a twoinch sapphire wafer at room temperature. A preferred method for theremoval of epitaxial film 19 from growth substrate 18 is the use of atleast one vertical scanned line 22 and at least one horizontal scannedline 25 to define isolated regions within a wafer as shown in FIG. 3B.By pre-lifting regions of the epitaxial film 19, higher yields can berealized in subsequent separation. The formation of stress crack 24 andor scribing 21 and 26 of nitride film 19 or growth substrate 18 tofurther reduced stress and allow for larger pieces of freestandingepitaxial layer 19 is disclosed.

FIG. 4 depicts a process for removal of thick epitaxial film 28 from agrowth substrate 29. In thick epitaxial films 28, significant backsidedeposition and edge growth can occur as depicted in FIG. 4A. This leadsto very high stresses on the outer perimeter of the overall wafer.

FIG. 4B depicts a thick epitaxial film 30 on a growth substrate 31.Removal of backside deposition and/or edge growth may be through, butnot limited to, mechanical means, masking during growth process, laserscribing and break, and chemical etching means. By removing the outeredge growth and backside deposition, improved yield in subsequentprocessing steps can be realized.

In FIG. 4C, heating element 32 may optionally be used to reduce bow oflayers 33 and 34 for the laser liftoff process or partial laser liftoffprocesses described previously. The use of a flexible heating element 32is a preferred embodiment. Using the properly designed template formedfrom layer 33 and 34 and the proper temperature of flexible heatingelement 32 it is possible to reduce the bow during liftoff and increaseyield. Alternately, irradiation, heated air and other means may be usedto heat layer 33 and 34. A temperature range of ambient to 500° C. maybe used. More preferably a temperature range of 100° C. to 300° C. maybe used. In addition, use of heating element 32 to flatten the bow suchthat a more uniform laser focus plane is created and liftoff or scribingcan be accomplished without cracking the wafer may also be utilized.Laser radiation 35 may consist of, but not limited to, a circular spot,line source, or a pattern that is scanned via movement of the wafer viastages or via movement of the beam via a galvo or other optical stage.Since the laser process is additive the use of randomized or multiplescans to create a given amount of liftoff in a particular region is alsodisclosed. Even more preferred is the use of a line source with a widthdimension less the 5 microns and a length greater than 100 microns. Thenarrowness of the line source is critical to prevent cracking of layer33. In all these applications the nitride layer 33 must be substantiallycrack free and exhibit a thickness greater than the width dimension ofthe line source to allow for reasonable yield. Layer 33 may consist ofpolar, non-polar, semi-polar, semi-insulating, doped, undoped andlayered nitrides. In particular, a preferred embodiment is layer 33consisting of tens of microns of undoped or doped GaN with a few micronsof semi-insulating or insulating GaN or nitride alloy as a growthsubstrate for FET and other electronic devices, which require anisolated base. The use of these semi-insulated or insulated freestandingnitride films in forming power and RF devices for communications andhybrid vehicles is also an embodiment of this invention.

FIG. 4D depicts a freestanding epitaxial film 37 and patterning step 36on said freestanding epitaxial film 37. In the case of thickfreestanding epitaxial films 37, areas greater than 1 cm2 can be createdwhich can be handled and processed without the need for a growthsubstrate described above. The removal of the growth substrate describedabove allows for access to both sides of the freestanding epitaxial film37. Preferred are freestanding epitaxial films 37 with thickness greaterthan 10 microns. More preferred are freestanding epitaxial films 37 withthickness greater than 30 microns due to increase handlability andcrystal quality. Also preferred are freestanding epitaxial films 37,which are less than 200 microns due to better thermal performance andcost. Patterning step 36 may be subtractive or additive.

The use of high temperature processing up to the decompositiontemperature of the freestanding epitaxial film 37 is disclosed. The useof ammonia and inert and/or vacuum atmospheres for processing is alsodisclosed especially for processes with temperature greater than 500° C.The use of laser scribing and feature forming for patterning step 36 isa preferred embodiment of this invention. In particular, the partialscribing through the wafer to allow for subsequent segmentation isdisclosed.

FIG. 4E depicts the use of coating means 38 over patterned epitaxialfilm 40. As disclosed previously, epitaxial growths on one or both sidesof patterned epitaxial film 40 is preferred. Alternately, coating means38 may include, but not limited to, amorphous growth, polycrystallinegrowths, dip coatings of organic and inorganic media. A preferredembodiment for coating means 38 is epitaxial growth on one or both sidesof patterned epitaxial film 40 of a transparent conductors including,but not limited to, doped ZnO alloys, doped nitride alloys, as well asother materials known in the art. ZnO alloys and nitride alloys are apreferred embodiment due to high thermal conductive, low absorption, andability to grown high quality crystalline layers.

The incorporation of luminescent elements in the form of ions including,but not limited to, Bi, Eu, Er, Dy, Pr, Li, Ho, Ce and other rare earthsto the freestanding nitride structure is also an embodiment of thisinvention. The formation of thick layers of these luminescent elementsto allow for sufficient absorption and reemission is preferred. Theformation of the luminescent layers on one or both sides may also bepart of the structure. Even more preferred is the use of two differentcoating means 38 on either side of patterned epitaxial film 40. In thismanner a wide range of the colors can be generated combining theemission spectrum of an LED in patterned epitaxial film 40 with theemission spectrum of the two different coating means 38 excited by theemission spectrum of an LED in patterned epitaxial film 40. Alternately,different absorption spectrum to two different coating mean 38 can beadded for solar applications. Even more preferred is a luminescent layerwhich converts the radiation of one wavelength to another wavelength fora coating means 38 on a patterned epitaxial film 40 and an absorbinglayer for a separate coating means 38 which allows more efficient energyconversion.

Also included is the use of patterned epitaxial film 40 as a growthsubstrate for luminescent layers for coating means 38. This may be doneusing but not limited to MBE, MOCVD, HVPE, sputtering, evaporation, dipcoating, spin coating, and/or LPE. The use of coating means 38 which arecomposites containing luminescent materials either in powder, quantumdots, or other luminescent means is disclosed. Coating means 38 may ormay not fill in feature 39 formed by the patterning step disclosedearlier. In this manner, efficient extraction elements can be formed.

Alternately, series resistance can be reduced and Vf can be reduced byusing coating means 38 to allow conductive layers to be embedded intopatterned epitaxial films 40. The resulting 3 dimensional partial viacan used to control current spreading by varying the density of featuresand depth of features.

FIG. 4F depicts the segmentation of epitaxial die 43 with coating means42. The use of breaking means including, but not limited to, mechanicalpressure, ultrasonics, and chemical means is disclosed. Edge die 41 maybe discarded or further processed to isolate the devices. Alternately,mechanical or chemical means may be used to remove coating means 42along the edges. In either case this process can be used to create butnot limited to solar cells, LEDs, EELEDs, Laser diodes, power devices,RF devices and MEMS.

FIG. 5A depicts a solar cell. A freestanding epitaxial film 44 of thepresent invention is coated with an absorptive layer 45 and an outercontact layer 46. The freestanding epitaxial film 44 may consist of, butis not limited to, alloys of nitrides and oxides. More preferably,freestanding epitaxial film 44 is a nitride alloy or zinc oxide alloy.

Absorptive layer 45 is grown on at least one side of freestandingepitaxial film 44. The function of absorptive layer 45 is to absorb andconvert incident radiation into solar energy while freestandingepitaxial film 44 and outer contact layer allow for extraction of thisenergy from the device as known in the art. More preferably, absorptivelayer 55 is a nitride or oxide alloy with a band edge within the visiblespectrum. The formation of multiple layers and/or photonic structureswithin absorptive layer 55 is to enhance conversion efficiency, tomodify the absorption spectrum and/or to enhance electrical propertiesof the device.

Outer contact layer 46 may be, but is not limited to, a nitride or oxidealloy. More preferably, outer contact layer 46 is substantiallytransparent to radiation to be absorbed by absorptive layer 55. Evenmore preferably, outer contact layer 46 is the opposite conductivitytype semiconductor material to freestanding epitaxial film 44 such thata solar cell is formed. Most preferred is that freestanding epitaxialfilm 44 is a n type nitride alloy and outer contact layer 46 is a p typenitride or oxide alloy.

The addition of current extracting elements including, but not limitedto, metal traces, thick film conductors, degenerative semiconductors tothe outer surface of contact layer 46 is disclosed. The formation ofsurface texturing on any of the layers disclosed to enhance absorptionincluding photonic crystal structures as known in the art is anembodiment of this invention.

FIG. 5B depicts a possible side view of the solar cell depicted in FIG.5A. In this particular embodiment, freestanding epitaxial film 44extends past absorptive layer 45 and outer contact layer 46 to exposecontacts 47 and 48. Isolation of contacts 47 and 48 from solar cell body49 can be created by, but is not limited to, etching, laser ablation,chemical means and/or mechanical means. The addition of a passivationlayer 50 is also disclosed. This passivation layer protects the varioussemiconductor layers from being shorted out by various environmentalfactors. The use of thick film and thin film metallization on contacts47 and 48 and at least partially over solar cell body 49 to facilitateand/or enhance current extraction and/or subsequent interconnect is alsodisclosed. Alternately, the device construction disclosed in FIG. 5 canalso be used to create a dual side LED structure, EELED, RF device andpower device by replacing absorptive layer 45 with an appropriate lightemitting and/or semiconducting active region.

FIG. 6 depicts an LED with an embedded contact spatially patterned toenhance current spreading. Even though the authors have disclosed theuse of degenerate epitaxially grown transparent conductors, there stillcan exist current spreading limitation especially at high currentdensity in large area die.

In FIG. 6A, top contact 51 forms an ohmic contact to contact layer 52,which then is electrically in contact to active region 53, which then inturn is in contact with contact layer 54. In the case of a vertical LED,contact layer 52 and 54 can be opposite conductivity semiconductortypes. More preferably, contact layer 52 and 54 are nitride and/or oxidealloys. Based on the use of thick contact layers 52 and 54, it becomespossible to modify the resistive nature of the layer using features 56.As an example, Si doped Gallium nitride typically has a resistivity ofapproximately 0.05 ohm-cm while degeneratively doped ZnO has aresistivity of less than 0.0005 ohm-cm. If features 56 are cut through asubstantial portion of contact layer 54, conductor 55 can be used tolocally lower the resistivity. Conductor 55 can consist of any materialwith a lower resistivity than the associated contact layer beingmodified by features 56. The 3-dimensional resistivity of both p and/orn type semiconductors can be modified using this technique. Features 56can be formed using etching, ablation and mechanical means as known inthe art. Most preferred is the formation of features 56 by laserpatterning either during liftoff and/or after liftoff from the growthsubstrate.

FIG. 6B depicts a spatially varying pattern of features 57 within LED 58that compensates for current crowding caused by the top contact 59. Inaddition to compensating for current spreading, the features 57 can alsobe used to enhance extraction efficiency. The use of metal contactsinstead of transparent conductive oxides is also disclosed. Mostpreferred is the use of low resistivity layer for conductor 55 thatfills or conforms to the features 57 such that overall device efficiencyis improved. Even more preferable is the use of epitaxial growth to formconductor 55 such that excess elements formed during laser patterning offeatures 57 diffuse into conductor 55 thereby reducing absorption lossesand enhancing ohmic contacts.

FIG. 7 depicts stacked LEDs with enhanced current spreading and/oruniform drive conditions. By combining LEDs with different outputspectrums and stacking them, broadband emitters are created.

FIG. 7A depicts three freestanding LEDs 60, 61, and 62. In thisparticular example, degenerative highly doped ZnO is used to allow forcontact between the three freestanding LEDs 60, 61, and 62. The use ofconductive epoxy, thick film conductors and other metal bearing contactmethods to enhance interconnect between the three freestanding LEDs 60,61 and 62 is an embodiment of this invention.

Contacts 64 and 65 provide for the injection of current into the devicewhile outer envelope 63 index matches, provides an environmental seal,and mechanically holds the assembly together. Unique with this inventionis the ability to use low temperature glasses to form outer envelope 63because of the high temperature nature of freestanding LEDs 60, 61, and62. The number of devices and electrical connection of the devices maybe varied to produce a desired light source. While three LEDs aredepicted, the use of more or less LEDs ranging from 1 up is disclosed.The use of anti-parallel, series, parallel and combinations ofanti-parallel, series, and parallel is also disclosed. The use of morethan two contacts 64 and 65 to facilitate AC, sequential, and/ortuneable control of the LEDs is also disclosed.

The inclusion of luminescent elements between and/or external tofreestanding LEDs 60, 61, and 62 may also be used. The inclusion ofluminescent elements within and/or on the outer surface of outerenvelope 64 may also be incorporated. The use of luminescent elementsand/or LEDs with different emission wavelength and combinations are allpossible with this invention.

FIG. 7B depicts voltage variation across an LED based on non-uniformgrowth conditions for two stacked LEDs. With this invention andfreestanding nitride films, it is possible to create LEDs and solarcells that have variable wavelength responses even within a given die.By stacking freestanding LEDs 60, 61, and 62, many non-uniformities canbe balanced out. The resulting volume emitter in the case of LEDs,volume absorber in the case of solar cells, volume diode in the case ofstacked diodes, and/or volume modulator in the case of a RF device is toenhance uniformity and increase device performance levels. In the caseof a volume emitter created by stacked LEDs, if absorption losses arelow as in this case, the flux per etendue 67 can be increased and thecurrent spreading losses 68 can be reduced. In the case of a volumeabsorber, current extraction 67 can be increased and the absorptioncross-section 67 can be increased.

The use of reflectors and other recycling means to further enhanceradiance or absorption are included as embodiments as well. For powerdevices, stand-off voltage, peak voltage and current uniformity based onvolume diodes can be enhanced. For RF devices the use of stacked devicescan greatly reduce inductance and capacitance effects. In FIG. 7B twonon-uniform LEDs, which have varying turn on voltages are used tocompensate for the other such that a uniform turn on voltage 66 iscreated across the device. Since these die also have different emissionwavelengths across each die broadband emitters can be formed.

This invention using thin freestanding nitride films permit elegantmeans to form high efficiency devices. For example, FIG. 8A depicts anEELED based on dual sided processing. Active region 74 for emission oflight may consist of gain media and/or and active emitter. Claddingcontacts 71 and 72 provide electrical contact to the active region butalso exhibit a lower refractive index such that a waveguide is formed.Top contact 70 and bottom contact 73 provide for electrical input andoutput to the device. A reflective mirror 75 is positioned such thatlight emitted within the active region is coupled back into the activeregion and re-emitted as output 69. The device is a side emitting LED.

FIG. 8B depicts the use of scribing lines 76 cut in freestanding film 78containing a gain media and/or active region 79 such that scribing lines76 align to cleavage planes within freestanding film 78. As suchindividual devices can be later cleaved as depicted by cleave lines 77forming a partial mirror for EELEDs, laser diodes, and other gain baseddevices.

FIG. 9 depicts a volume emitter utilizing the freestanding nitride filmof this invention with a restriction in output. FIG. 9A depicts a volumeemitter consisting of a stack of freestanding LEDS 80, 81, and 82.Alternately, this stack could also contain at least one luminescentelement as previously disclosed. In either case, a volume of emission iscontained or restricted in some manner such that some portion of theemitted light is recycled back through the origin of the emission. Aspreviously stated recycling can be done via restriction in emissionarea, restriction of angular output, restriction of polarization andrestriction of wavelength. In FIG. 9A reflector 83 restricts theemission area 84. Enhancement of radiance based on restricting theemission area 84 such that the emission area 84 is less than the area ofone of the freestanding LEDS 80, 81 and 82.

FIG. 9B depicts a volume emitter utilizing the freestanding nitride filmof this invention with a combination of area restriction created bycontacts 85 and 86 and/or an angular restriction 87 and 88 including butnot limited to BEF, microoptical elements, diffractive elements,photonic crystals, reflective polarizers, and dichroics such that aportion of the emitted light from at least one of the emitters withinthe volume emitter recycles back to the original source of emission.

FIG. 10A depicts an LED display based on at least one epitaxial linearsource 90 with addressing means. Using the techniques described abovelinear arrays of LEDS can be formed which can be individually addressedusing a simple interconnect. Features 89 can be used to isolateelectrically and/or optically the individual elements.

FIG. 10B depicts the use of interconnect films 91 and 95 and contacts 92and 94 to electrically interconnect the linear array elements. Adhesivelayer 96 can be used to electrically and/or optically isolate theindividual elements. Features can be used to cause cleavage at cleavelines 93 after assembly. This enables enhanced optical isolation of theindividual elements. The stacking of these linear arrays to form areaarrays, which are addressable, are also an embodiment of this invention.

FIG. 11 depicts a white light source based on stacked epitaxial chipsformed by the methods of this invention to form volume emitters andsolid luminescent elements. The ability to form volume emitters can leadto much higher flux density than simple area emitters. The use of solidluminescent elements are required to prevent not only degradationeffects but also thermal quenching effects. A number of ceramic, singlecrystalline, polycrystalline and composite luminescent materials havebeen developed. The use of these materials with volume emitters 99 is anembodiment of this invention. More preferred is the use of at least oneelectrically conductive luminescent element 97 for the ejection and/orextraction of current into the volume emitter 99.

Alternatively, the shaping luminescent element 97, whether electricallyconductive or not, and its use with a volume emitter is an embodiment ofthis invention. A preferred embodiment is the use of nitride and/orelectrically conductive oxides, in particular zinc oxide alloys asluminescent element 97.

Alternately, element 98 may consist of but not limited to reflectiveelement, output sensor, additional luminescent element, cooling means,absorber, and/or gain media being pumped by volume emitter. Anadditional bottom reflector 100 is disclosed to optically restrict lightoutput from volume emitter 99 and/or provide a thermal conduction pathto heatsink 101.

FIG. 12 depicts an LED powder containing at least one glass frit. Thefreestanding LEDs 103 can be processed at atmospheric temperaturesgreater than 700 degrees C. due to their intrinsic properties. The LEDemitting layers may be formed by the methods previously disclosedherein. They can be processed at higher temperature using controlledatmosphere such as, but not limited to, vacuum, inert, and ammonia.These high processing temperatures enable the use of brazing, frits,glasses, conductive thick films, and glazes typically not permissiblewith conventional LEDs. In addition, the ability to form a verticalstructure with sufficient thickness and mechanical integrity to behandled enable the formation of composites as depicted in FIG. 12.

Freestanding LED 103 is dispersed within a matrix 102 which may consistof, but is not limited to, low temperature glasses, organic binders,photo imaging matrixes, liquids and other thick film materials. Theresulting material may be subsequently processed via spin coating,dipping, printing, ink jet dispensing, spraying, tampo, transfer tapes,doctor blading, and other thick films processing techniques as known inthe art. The use of this LED powder in liquid, powder, solid and gaseoustransport processes is also disclosed. A preferred embodiment is thatthe freestanding LEDs are thicker than the largest sized matrix 102particle.

Alternately the matrix 102 may be a film to which the freestanding LEDs103 are temporarily mounted. Upon melting of the matrix 102, thefreestanding LEDs 103 are embedded within said matrix 102. The additionof luminescent elements within the matrix 102 and/or attached tofreestanding LEDs 103 is also disclosed. These luminescent elements mayconsist of, but are not limited to, quantum dots, powders, ceramics,composites, glasses, and single crystal elements. The freestanding LEDs103 preferably are greater than 10 microns thick and have an area tothickness ratio greater than 1. The glass matrix 102 preferably has amelt point greater than 300 C and provides an environmental seal toprotect the freestanding LEDs 103.

FIG. 13 depicts a method of forming a light source using an LED powder.The LED powder described above is sandwiched between top contact 104 andbottom contact 108. Additionally, external contacts 109 and 105 may beadded to allow for additional levels of interconnect including but notlimited to soldering, brazing, welding, and mechanical contacts.Freestanding LEDS 106 act as spacers between top contact 104 and bottomcontact 108. Preferably, at least one of top contact 104 and/or bottomcontact 108 contains at least one transparent conductive layers throughwhich current can to delivered to the freestanding LEDs 106. In the caseof non LED devices, the use of opaque and/or metal top contact 104 andbottom contact 108 is also disclosed. The use of volume emitters, whichhave been pre-assembled either using wafer bonding techniques or otherbonds means is also disclosed. The matrix 107 may include but notlimited to, thermoplastics, thermosets, low temperature glasses,epoxies, luminescent elements, directive elements, high temperatureglass spacers, and composites. Most preferably, the matrix 107 consistsof a glass frit with a melting temperature below the decompositiontemperature of the freestanding LEDs 106. The application of pressure,vibration, ultrasonics, heat, and radiation to melt and/or otherwisecure the matrix 107, such that a sealed assembly is formed containingfreestanding LEDs 106, electrically connected together is disclosed. Inthe case of non-LED applications, the use of this approach to formhermetically sealed arrays of devices is also disclosed.

In the following we show a method to process the freestanding nitridefilms and perform dual sided processing. FIG. 14A depicts a freestandingnitride film 110 with an optional absorbing layer 111 and variouscooling and heating means. Typically a susceptor or platen is used toheat and cool nitride based devices. This however leads to large thermaltime constants due to thermal inertia. Freestanding nitride film 110however has a very low thermal mass and is substantially homogenous, assuch much faster thermal ramps can be employed in the fabrication ofdevices using freestanding nitride films 110 than templates, non-nativesubstrates or even bulk nitride wafers. Freestanding nitride film 110and optionally absorbing layer 111 have a temperature determined by thebalance of input energy 117 and 118, convective cooling 116 and 115, andradiative cooling 113 and 114. Convective cooling 116 and 115 areinfluenced strongly by convective flow 112 within the reactor ordeposition equipment. By minimizing the thermal mass of freestandingnitride film 110 and optionally absorbing layer 111 it possible todramatically decrease growth times which can not only lead to lowermanufacturing costs, but better performance devices by improvedepitaxial layer definitions. Both single sided and dual sideddefinitions are disclosed. FIG. 14B depicts a mounting configuration inwhich support means 120 have a minimum impact on the thermal timeconstant of the freestanding nitride film 119 and also enable dual sidedgrowth. In general, support means 120 hold the outer upper and lowersurface edges of the nitride film. The support means do notsubstantially increase the thermal time constant of the freestandingnitride film 119 is a preferred embodiment of this invention.Freestanding nitride film 119 with a thickness less than 100 microns ispreferred due to the relationship τ=ρc_(p)V/hA_(s) which defines theconvective thermal time constant τ of a surface. As can be seen by theequation for simple films, the ratio V/A_(s) has a strong influence onthe time constant of freestanding nitride film 119 and is essentiallyequal to the thickness of the film. The τ thickness should be minimized,the convective thermal coefficient h should be maximized and the ρc_(p)should be mimimized. The convective thermal coefficient h is tied toreactor growth conditions to some extent and is limited to under 100W/m2K for most gas based systems. By using a freestanding nitride filmρc_(p) is minimized compared to conventional processes using thicksapphire, silicon, and/or SiC substrates. Therefore a preferredembodiment of this invention is the use of freestanding nitride film 119with a convective thermal time constant less than 3 seconds.

FIG. 15A depicts an alternate low thermal mass mounting for afreestanding nitride film 121. In this configuration a hole is cut infreestanding nitride film 121 such that a pin or wire 124 may be used tosupport the freestanding nitride film 121. Optionally, additionalsupport means 122 and 123 can be used to provide additional support tothe freestanding nitride film 121. Or optionally posts 122 and 123 maybe used without the hole in the film to support it by pinching.

FIG. 15B depicts a typical InGaN quantum well growth cycle. Becausecomposition is strongly dependent on growth temperature, the temperatureof the freestanding nitride film determines the composition determinesthe composition to a great extent. As an example, a typical quantum wellmight contain 30 Angstoms of In_(0.03)Ga_(0.07)N and 120 Angstroms ofGaN with a typical growth rate of 0.1 microns/hour. To grow the quantumwell, the freestanding nitride film will be ramped form startingtemperature 128 to InGaN growth temperature 129 which may be around 750°C. Depending on the previous parts of the structure the startingtemperature 128 can be between ambient and 1200° C. In any case there isthe transition time between these two temperatures, which is ramp time127. Similarly ramp times 125 and 126 define the time it takes to changethe temperature of freestanding nitride film within the reactor. GaNtemperature 130 will be typically 1000° C. As such in order for the ramptimes 127, 125, and 126 to not represent a significant percentage of thegrowth time very low growth rates must be used to ensure at least themajority of the 30 angstroms and 120 angstroms layer are being grownusing the right composition. In commercial reactors utilizing largewafers, growth rates less than 0.1 microns/hour are required simply toapproximate the desired composition and layer thickness. By utilizingsmall, thin freestanding nitride films of this invention the thermaltime constants of the reactor system ramp and process times can beminimized leading to more clearly defined structures within theepitaxial layers. If 0.1 microns/hour is used for a 30 Angstrom quantumwell the total growth time for the InGaN layer is approximately 100seconds. By decreasing the thermal mass of the growth substrate asdescribed above, enabling cooling and heating from both sides of thesubstrate, and minimizing the thermal mass of the supporting structuresholding the growth substrate ramp times can be reduced which in turnenables higher growth rates which decrease manufacturing costs as well.Lastly, the use of a substantially homogenous freestanding nitride filmalso is an important attribute of this invention because it enablesreduction of ramp times without cracking of the growth substrate.Template and non-native growth substrates are especially susceptible tothis problem due to higher built in stresses. A preferred embodiment ofthis invention is the use of freestanding nitride films, supportstructures, and heating/cooling means, which enable ramp rates greaterthan 250° C./sec during growth. In this manner higher growth ratesand/or better interface definition can be realized than conventionaltemplate, non-native and bulk wafer approaches.

FIG. 16 compares the thermal time constant of various substrates. Thethermal time constant to a great extent determines how quickly thegrowth conditions within the reactor can be changed. While the exactnumber requires extensive modeling, FIG. 14 illustrates how the thinnature of freestanding nitride foils can have a dramatic impact ongrowth conditions. In the case of MQW growth layer thicknesses vary fromtens to hundreds of Angstroms. This mandates that either very slowgrowth rates are used or very rapid changes in reactor growth conditionsare required. In conventional large platen reactors the thermal mass ofthe substrates and associated platens limit how rapidly the temperaturecan be changed. As such typical growth times of 10 minutes per QW areused. Freestanding nitride films or foils can reduce thermal timeconstants by more than 10×, which then allows for much faster QWgrowths. This reduces the overall manufacturing time significantly. Inaddition, very rapid thermal ramps are enabled by freestanding nitridefoils, which can be used to more precisely control the interfacesbetween various layers. As an example, a single QW consisting of 30Angstroms of InGaN and 120 Angstroms of GaN at an average growth rate of0.1 microns/hour (0.3 Angstroms per second) would take 108 seconds togrow the 30 Angstrom layer. At 1.0 microns/hour it would take 10 secondsto grow the 30 Angstrom layer. During this period temperature may bevaried as much as 300 C along with significant changes in reactor gases.If it takes 20 to 30 seconds to cool the substrate temperature it isapparent that high growth rates have a problem with accuratelydepositing the desired composition within the layer. It is disclosedthat freestanding nitride foils can be used to overcome this problem. Assuch freestanding nitride foils with volume to area ratios less than0.01 cm are an embodiment of this invention. Even more preferred the useof freestanding nitride foils with convective thermal time constant lessthan 1 sec when h is between 1 and 100 W/m2C within a MOCVD, HVPE, orMBE system is disclosed. The use of these foils to create LEDS, solarcells, laser diodes, RF devices, photoelectrochemical cells and powerdevices is also disclosed.

FIG. 17A depicts a typical complete growth cycle for a MQW LED. Thegrowth cycle can be divided into 3 main regions, nucleation and n layergrowth 131, MQW growth 132, and barrier and p layer growth 133. By usingfreestanding nitride film, growth 131 can be eliminated and the reactorcan be optimized for MQW growth 132 and barrier p layer growth 133.Using prior art conventional methods with large wafers, complete growthcycle can take up to 8 hours with the majority of the time spent innucleation and n layer growth 131. Additional layers may be added toimprove crystal quality or control stresses prior to MQW growth 132.This invention shows how freestanding nitride films with low thermalmass can dramatically reduce this growth cycle times. In addition FIG.17B depicts how low thermal mass approaches can be used to enable betterdefinition of the layers themselves through the use of rapid growthinterruptions 134 between layers. Such an approach in conventionalreactors becomes prohibitively expensive. In addition, because processgases cannot be instantly changed within the reactor, composition ofeach layer are graded and exhibit tails into next layer. By decreasingthe thermal time constants, it becomes possible to more cleanly andaccurately define the composition, thickness and interfaces betweenlayers. The use of growth interruptions 134 in a rapid thermal growthreactor using freestanding nitride films is a preferred embodiment ofthis invention.

FIG. 18A depicts a multi chamber reactor 138 and 139 to further decreasetime constant of layered structures. Freestanding nitride film 135 isshuttled from chamber 138 through port 137 into chamber 139. As anexample MQW growth may occur within chamber 138 and barrier and p layergrowth may occur within chamber 139. Alternately, InGaN layer may occurin chamber 138 and GaN layer may occur within chamber 139. MQWs would becreated by shuttling back and forth between the two chambers. Theaddition of more chambers is also disclosed. FIG. 18B depictsfreestanding nitride film 143 and Ga side 141 and N side 142. In all theprevious disclosure it should be noted that growth rates and compositionis effected by whether Ga side 141 or N side 142 is being grown on. Theformation of dual sided devices where the different outputs or deviceperformance is created based on Ga and N deposition differences is alsodisclosed.

FIG. 19A depicts a freestanding nitride film 145 with multipledepositions 143, 144, 146, and 147 on both sides. The growth of multiplelayers on both sides of at least one freestanding nitride film 145 is apreferred embodiment of this invention. FIG. 19B depicts howfeedthroughs and traces 150 can be used to interconnect multiplefreestanding freestanding nitride films 148 and 151. Optionally,insulative and semi-insulating layers 149 and 152 may be used to isolateelectrically, optically, and mechanically bond the films together.

FIG. 20 depicts a freestanding nitride film 153 in which multipleepitaxial layers 155 and 154 are grown within a hole in freestandingnitride film 153. The hole may be formed via chemical means, mechanicalmeans, and laser ablation means. Secondary etching to expose cleancrystal planes within the hole may also be used. An interconnect means156 may optionally be used to form a two or three terminal device withthe freestanding nitride film 153 forming one side of the device.Optionally excess growths 157 on one or both surfaces of freestandingnitride film 153 may be removed by mechanical, CMP, and laser ablation.

A preferred process of making a LED, solar cell or microelectronicdevice utilizing the following steps: (1) Growing low defect density GaNor other useful nitride or oxide layer to a thickness of >10 μms, morepreferably ≧20 μm, and, most preferably, ≧30 μm, on a sapphire substrateusing HVPE for GaN and/or MOCVD for the nitride layer, optionally wheresaid nitride film has a defect density less than 10E8 cm2. (2) Liftingoff said GaN or nitride layer using a DPSS laser with the scanningtechniques described previously herein. The lifted off GaN issimultaneously sectioned into square or rectangular foils of \sufficient size to be used as growth substrates for fabricating aplurality of devices on them preferably less than 9 cm2, more preferablyless than 6.5 square centimeters, and most preferably ≧4 square mm. Thethickness of these freestanding pure nitride or oxide substrates is atleast 10 μm, preferably at least 20 μm, and more preferably ≧30 μm suchthat they survive the liftoff process but are also robust enough tohandle. (3) Optionally, prior to lifting of said nitride a first devicemay be grown or fabricated on the top surface in order that a devicewith different or complimentary characteristics may be grown on theunderside after lifting off. (4) These relatively thin nitride or oxidefreestanding substrates are then processed in a suitable reactor, morepreferably a rapid thermal reactor that is optimized to processfreestanding semiconductor substrates of relatively small sizepreferably less than 9 cm2, more preferably less than 6.5 squarecentimeters, and most preferably ≧4 square mm. Said reactor is capableof growing HVPE or MOCVD processes and optionally capable of growing onmultiple films simultaneously. (5) Devices may be grown on one or bothsides of these films in said reactor. To process on only one side acoating may be applied to one side prior to processing or by clampingthe film against a platen to restrict growth to the exposed side. Togrow on both sides, the freestanding film is secured in the reactor suchthat both sides are exposed to the growth process.

While this invention has been described in conjunction with the specificembodiments outlined above, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, the preferred embodiments of the invention as setforth above are intended to be illustrative, not limiting. Variouschanges may be made without departing from the spirit and scope of theinvention as defined in the following claims.

1. A method of forming a semiconductor device comprising the steps of:depositing a first semiconductor layer on the upper surface of afreestanding semiconductor alloy film; said first semiconductor layerbeing an oxide, a nitride, silicon, an antimonide, a metal, a dielectricor other semiconductor material; said semiconductor alloy being anitride alloy or a zinc oxide alloy; said depositing being by adeposition process of MOCVD, HVPE, MBE, CVD, sputter, evaporative, orspincoating; and, depositing a second semiconductor layer on the lowersurface of a freestanding semiconductor alloy film; said secondsemiconductor layer being an oxide, a nitride, silicon, an antimonide, ametal, a dielectric or other semiconductor material; said semiconductoralloy being a nitride alloy or a zinc oxide alloy; said depositing beingby a deposition process of MOCVD, HVPE, MBE, CVD, sputter, evaporative,or spincoating; and further comprising the steps of: growing saidfreestanding semiconductor alloy film on a substrate; patterning saidfreestanding semiconductor alloy film; prior to said depositing thefirst semiconductor layer on the upper surface of said freestandingsemiconductor alloy film and prior to depositing the secondsemiconductor layer on the lower surface of said freestandingsemiconductor alloy film; and removing said freestanding semiconductoralloy film from said substrate.
 2. The method of forming a semiconductordevice of claim 1 wherein said removing said freestanding semiconductoralloy film from said substrate is by scanning laser pulses through saidsubstrate to said freestanding semiconductor alloy film.
 3. The methodof forming a semiconductor device of claim 2 further comprising the stepof: heating said freestanding semiconductor alloy film and saidsubstrate.